Ruby on Rails and prepated statements

It is knows that Ruby mysql driver supports prepared statements.
Why do not active record adapters support them? Is there any reason
for it?

Well, ActiveRecord sort of *does* support prepared statements, in the
sense that parameterized queries are a type of prepared statement, and
ActiveRecord uses parameterized queries. Are you looking for
precompiled prepared statements? If so, perhaps stored procedures
would be the way to go (although mySQL's procedural language sucks)?